The present invention provides a method for determining the sea floor roughness using multibeam echosounder. The present invention also provides a method to precisely determine the beam incident angle at the sea-bottom and in consequence, acoustic backscatter data to be unambiguously related to the nature of the sea-bottom.
Multibeam sounding systems are widely used for bathyretric mapping and also have potential to provide quantitative backscatter information which can be used to classify the seabottom roughness. Because of narrow multibeam geometry, the angular dependence of the acoustic backscatter can be derived, and accurate quantitative estimate of backscatter is possible along with bathymetry. The changes in the backscatter strength for different survey areas can be compared for the same incidence angles using the shape parameters of the backscatter curves. Before initiating any model study using multibeam angular backscatter information, corrections in terms of system gain, is an important step to be carried out. Using multibeam bathymetry, accurate measurement of the bottom slopes is also possible, which can be used to compute backscatter strength for the determination of correct angle of incidence. Also, bottom insonification area normalization is an important factor to be employed before carrying out any seabottom characterization.
The multibeam systems have larger seafloor coverage when compared to the single beam system. Hence, in coming future many fold increases in the use of multibeam system is expected in comparison with the single beam system. In the multibeam processing system, the received electrical envelopes at the bottom echo processor are sampled and converted to rms voltages. Techniques for seafloor classification are still maturing and multibeam backscatter data and bathymetry data will play a significant role in the development of these techniques.
A reference may be made to Anonymous, (1993), xe2x80x9cAtlas Hydrosweep DSxe2x80x94Interface specifications: Magnetic Tape Recording and data for Backscattering Analysisxe2x80x9d, STN Atlas Elektronik BmgH, Bremen.
A reference may be made to De moustler, C. and Alexaondrou, D., (1991). Angular dependence of 12 KHz. Seafloor acoustic backscatter . Jour. Acoust. Soc. America., 90: 531-533.
A reference may be made to Jackson, D. R, D. P. Winebreuner, and A. Ishimaru, (1986). Application of the Composite roughness model to high frequency Bottom backscattering, Jour. Acoust. Soc. America., 79: 1410-1422.
A reference may be made to Anonymous, (1999), xe2x80x9cCruise Report of OVR Sagarkanys-Cruise Number SK-140 -Bxe2x80x9d, National Institute of Oceanography, Dona Paula, Goa, India.
A reference may be made to Talukdar, K. K. and R. C. Toyce, (1 991), xe2x80x9cDigital Processing of sidescan images from bottom backscatter data collected by Sea Beamxe2x80x9d, Mar. Geodesy, 14, 81-100.
The main object of the present invention is to provide a method for determining seafloor roughness using multibeam echosounder.
The present invention provides a method for determining sea floor roughness using multibeam echosounder, said method comprising providing backscatter, bathymetry and multibeam operational parameters; removing system gain from the backscatter data to obtain real seafloor backscatter data; calculating across track seafloor slope; incorporating correction factor in actual beam arrival angles; calculating seafloor area; measuring backscatter strength; obtaining backscatter strength data at 1xc2x0 angle; obtaining power law parameters; using the above-obtained parameters in a seafloor roughness power law model; splicing the multibeam backscatter data; including sediment volume roughness model; and verifying the validity of the composite roughness model.
Accordingly, the present invention provides a method for determining sea floor roughness using multibeam echosounder, said method comprising:
(i) providing backscatter, bathymetry and multibeam operational parameters;
(ii) removing system gain from the backscatter data to obtain real seafloor backscatter data;
(iii) calculating across track seafloor slope;
(iv) incorporating correction factor in actual beam arrival angles;
(v) calculating seafloor area;
(vi) measuring backscatter strength;
(vii) obtaining backscatter strength data at 1xc2x0 angle;
(viii) obtaining power law parameters;
(ix) using the above-obtained parameters in a seafloor roughness power law model;
(x) splicing the multibeam backscatter data;
(xi) including sediment volume roughness model; and
(xii) verifying the validity of the composite roughness model.
In an embodiment of the present invention, the primary backscatter parameters recorded by the system are echo rms voltage and signal duration for each beam.
In yet another embodiment of the present invention, the overall signal processing gain (VTot) consists of matching circuits gain (VMatch), range related gain or Time varied correction gain (VTVC), beamformers gain (VBF) and filter gain (VFilt).
VTot=VMatch+VTVC+VBF+VFiltxe2x80x83xe2x80x83(I)
In still another embodiment of the present invention, the matching circuit gain is the gain provided by each transducer group.
In one another embodiment of the present invention, the range related gain or Time Varied Correction Gain is calculated using equations given in Anon, 1993.
In one more embodiment of the present invention, the range related gain consists of gain compensation due to signal travel through the water media and attenuation corrections and starting gain.
In an embodiment of the present invention, the starting gain VTVC comprises of preamplifier gain (Vv), user selected TVC (Vcorr) and system related offset gain (Voff).
In another embodiment of the present invention, the across track seafloor slope is calculated using bathymetry data.
In yet another embodiment of the present invention, the across track seafloor slope is calculated to incorporate correction factor in actual beam arrival angles.
In still another embodiment of the present invention, the bottom slope values with respect to each beam are computed using tangent inverse of the ratio between depth and lateral distance difference.
In one another embodiment of the present invention, the seafloor area is calculated based upon angle of incidence and bathymetry.
In one more embodiment of the present invention, the bottom sea surface area is calculated using the formula
A=Ab exp[xe2x88x92(xcfx86/xcfx86x)2]+Ap{1xe2x88x92exp[xe2x88x92(xcfx86/xcfx86x)2]}xe2x80x83xe2x80x83II
In an embodiment of the present invention, the actual echo root mean square level is calculated using DeMoustier and Alexandrou Formula (1991).
 less than 20 log10 RL(xcfx86) greater than =SL+ less than 10 log10 SB(xcfx86) greater than xe2x88x9240*log10 (R)xe2x88x922xcex1bR+10 log10 Axe2x80x83xe2x80x83III
In another embodiment of the present invention, the backscatter strength data at 1xc2x0 angle is obtained by bining each beam at 1xc2x0 angular bins and averaging the number of samples in each bin for steady areas.
In yet another embodiment of the present invention, the power law parameters are obtained by reading the measured backscatter data and subjecting it to curve fitting.
In still another embodiment of the present invention, Helmholtz-Kirchhoff approximation is used to determine interface roughness of the seabed.
In one another embodiment of the present invention, the splicing is done using Helmholtz-Kirchoff and Rayleigh-Rice theories.
In one more embodiment of the present invention, the incidence angle of 20xc2x0 is used for splicing.
In an embodiment of the present invention, the sediment volume roughness inodel is created by curve fitting the entire measured angular backscatter data.
In another embodiment of the present invention, matching the theoretical curve with the measured backscatter data does the verification.
Seafloor roughness studies using bathymetry provides geometrical shapes of the seafloor features. The determination of the texture aspect together with the seafloor features may be made by use of multibeam backscatter signal. Estimated seafloor roughness related parameters using modeled angular backscatter data are more important than bathymetry data. Use of composite roughness theory based model of Jackson et al, (1986) along with the processes angular backscatter strength has ample scopes.
High acoustic frequency (10 kHz-100 kHz) based profiling of the seafloor roughness include two basic types of roughness. They include water to seafloor interface roughness and sediment volume roughness parameters. Through use of power law parameters, determination of the parameters like: large scale roughness parameters of the seafloor i.e., root mean square(rms) slope and Radius of curvatures may be made. Similarly, small-scale roughness of the surveyed seafloor determined which is designated as rms relief height. The estimated volume roughness parameter is a ratio between the sediment volume scattering and attenuation coefficient is determined using this model. Based on the above computed results using estimated power law parameters, different survey areas can be systematically characterized.
The present invention provides a system of backscatter echo data processing and seafloor roughness estimation (power law parameters) providing appropriate theories to deal with the newly developed expanded multibeam data sets.
The approach adopted in the present invention for estimating water to seafloor interface roughness is to use the Helmholtz-Kirchoff interface scattering theory up to 15xc2x0, and the more relevant Rayleigh-Rice perturbation theory for incidence angles beyond this. The splicing technique adopted between the theories is an indicator of the success of the composite roughness theory using multibeam.
Scattering beyond 20xc2x0 is a composite of scattering from water-seafloor interface and volume scattering from sediments. The composite roughness theory can successfully be used to determine sediment volume roughness using expanded form of multibeam system and the presently proposed system.
Development of on-line system is underway to obtain seafloor roughness classification in real time. These systems can provide useful support to marine geologist to make instant decisions about the seafloor sediment sample collection and considerable savings of time.